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Philippe Gournay

A Very Low Bit Rate Protection Layer to Increase the Robustness of the AMR-WB+ Codec against Bit Errors. Philippe Gournay. Université de Sherbrooke Département de Génie Electrique et Informatique Sherbrooke, Québec, Canada. Outline. Context The AMR-WB+ Codec Bit Sensitivity Study

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Philippe Gournay

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  1. A Very Low Bit Rate Protection Layer to Increase the Robustness of the AMR-WB+ Codec against Bit Errors Philippe Gournay Université de Sherbrooke Département de Génie Electrique et Informatique Sherbrooke, Québec, Canada

  2. Outline • Context • The AMR-WB+ Codec • Bit Sensitivity Study • The Protection Layer • Evaluation Results • Audio Demonstration • Conclusion

  3. Context • Challenging Applications for AMR-WB+ • Streaming and Multimedia Messaging in GSM, DVB-H Mobile TV, Digital Radio • Channel Impairments • Lost Packets • Concealment Procedure • Bit Errors • Channel Coding (Forward Error Correction) • Inherent Robustness of the codec • Additional Protection Layer

  4. Overview of the AMR-WB+ codec • Speech and Music • Mono and Stereo Inputs • Wide Range of Audio Bandwidth • From 8 to 48 kHz • Wide Range of Bit Rate • 6 to 36 kbps in mono, 7 to 48 kbps in stereo • Hybrid Coding Model • Time (ACELP) and Frequency (TCX) Domain

  5. The AMR-WB+ Encoder PACKETIZATION ACELP 1 frame Bitstream Audio TCX Mode Selection 1, 2 or 4 frames Mode Index, ISF

  6. AMR-WB+ Frame Structure • Three out of the 26 possible ACELP/TCX coding configurations (a) ACELP ACELP ACELP ACELP (b) Short TCX ACELP Medium TCX (c) Long TCX One super-frame = 80 ms @ 12.8 kHz

  7. AMR-WB+ Embedded Bitstream Structure • For medium and long TCX, the packetization procedure is responsible for distributing the bitstream among the required number of packets. Core Mode Core Bitstream (ACELP or TCX) Stereo Mode Low-band Mid-band BWE (mono/right) BWE (left) Core codec Stereo extension

  8. Bit Sensitivity Study • Determine Individual Bit Sensitivities • Prior Knowledge (ACELP, AMR-WB Standard) • SNR Measurements • Bit Sensitivity Classes • Class A: Highly sensitive • Class B: Moderately Sensitive • Class C: Not Sensitive • Useful for U.E.P. (Unequal Error Protection)

  9. Segmental SNR (8.67 kbps mono) 100 100 80 80 60 60 SNR (dB) ACELP TCX20 40 40 20 20 0 0 50 100 150 200 50 100 150 200 Bit position 100 100 80 80 60 60 SNR (dB) TCX40 TCX80 40 40 20 20 0 0 100 200 300 400 200 400 600 800 Bit position

  10. The Protection Layer • Additional Protection Layer • Standard AMR-WB+ Payload Kept Unchanged • Very Low Bit Rate • 16 bits per frame (480 bits @ 32 kbps stereo) • Performs Error Detection and/or Correction • Class A: Error Detection and Correction • Class B: Error Detection • Class C: Nothing • Depends on the Core and Stereo Modes

  11. Core Protection Layer (1)

  12. Core Protection Layer (2)

  13. Stereo Extension Protection Layer

  14. Subjective Evaluation(32 kbps stereo) Note: bit errors are also applied to the protection layer.

  15. Audio Demonstration • AMR-WB+ @ 32 kbps stereo • Mode Index=40, ISF=1.333 • Original File • Clear Channel • Standard Codec, 0.5% BER • Modified Codec, 0.5% BER

  16. Audio Demonstration • AMR-WB+ @ 32 kbps stereo • Mode Index=40, ISF=1.333 • Original File • Clear Channel • Standard Codec, 0.5% BER • Modified Codec, 0.5% BER

  17. Audio Demonstration • AMR-WB+ @ 32 kbps stereo • Mode Index=40, ISF=1.333 • Original File • Clear Channel • Standard Codec, 0.5% BER • Modified Codec, 0.5% BER

  18. Audio Demonstration • AMR-WB+ @ 32 kbps stereo • Mode Index=40, ISF=1.333 • Original File • Clear Channel • Standard Codec, 0.5% BER • Modified Codec, 0.5% BER

  19. Conclusion • AMR-WB+ Bit Sensitivity Study • Sensitivity Classes A, B and C • Useful for U.E.P. • Additional Very Low Bit Rate Protection Layer • 16 bits per frame • Error Detection and Correction • Enhance Robustness against Bit Errors • Significant Subjective Quality Improvement • Decoded Audio is Free of Major Artifacts at a Significant 0.5% BER

  20. Thank you for your attention!

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